The present invention relates to rock supporting or reinforcing structure and, more particularly, to an improved combination of structure including a bolt threaded at both ends, and methods of installation thereof, to achieve improved tensioning of the bolt within the drill hole and thus improved support of the surrounding rock structure.
In mine work, such as coal mining, or in underground formations such as tunnels or other excavations, it is necessary to reinforce or support the roof and/or walls of the excavation to prevent rock falls or cave-ins. Among the most common means presently in use for effecting such support are elongated bars or bolts which are inserted into blind drill holes and anchored therein to hold a metal support or bearing plate in tight engagement with the roof or wall surface. Anchoring means within the drill hole normally comprise a mechanical expansion anchor including an expansible shell and a camming plug threadedly engaged on the end of the bar inside the drill hole, and/or a hardenable resin or other grouting.
In addition to anchoring a first end of the bar in the drill hole, it is also desirable that the bar be placed in tension in order to reinforce the rock formation surrounding the excavation. Tensioning means employed in the prior art include a system wherein the bar is in the form of a bolt threaded for some distance from each end. A mechanical expansion anchor is threadedly engaged with the end of the bolt inside the drill hole and a tensioning nut is threadedly engaged with the other end, outside the drill hole. The tensioning nut bears against a washer which is inserted over the end of the bolt between the tensioning nut and the bearing plate. In the intended manner of installation, torque applied by a wrench to the tensioning nut rotates the bolt and causes the camming plug to travel down the threads on the end of the bolt within the drill hole and expand the shell radially into tight engagement with the bore hole wall. Continued torque applied to the tensioning nut after the expansion anchor is set causes the tensioning nut to travel up the threads on the end of the bolt outside the drill hole, thereby tensioning the bolt and placing the surrounding rock structure in compression.
A major problem which has been encountered in the installation of such support/reinforcement systems is the tendency of the tensioning nut to travel up the threads on the bolt before the expansion anchor is expanded within the drill hole. That is, the initial torque applied to the tensioning nut, rather than causing rotation of the bolt and expanding the anchor, causes the tensioning nut to travel to the end of the threads with which it is engaged. Further application of torque will then expand the anchor, but the tension nut cannot travel further to tension the bolt after the anchor is fully expanded.
It is a principal object of the present invention to provide novel and improved means and methods of installation of mine roof support systems of the type wherein a bolt threaded at both ends has a mechanical expansion anchor on one end within a drill hole and a tensioning nut on the other end, outside the drill hole.
A further object is to provide means and methods for use in the foregoing type of installations which improve operation of the system without significantly increasing the cost thereof.
More specifically, the object of the invention is to provide a combination of structural elements, and methods of use thereof, which ensure that a mechanical expansion anchor on one threaded end of a mine roof support bolt is expanded to engage the wall of a bore hole before a tensioning nut travels up threads at the other end to tension the bolt.
Other objects will in part be obvious and will in part appear hereinafter.